A cellular radio system uses a grid of small service cells (or service zones) and a specific pattern of channel use within the group of service cells so that channels can be reused in a controlled way. A call in a cellular system is routed to a mobile unit via land-line trunks to a cell site (i.e., a stationary transmitter) in the vicinity of the mobile unit. Low-power RF transmission from the cell site is used for the last few miles. System logic locates an active user within the grid in order to hand off control of the call to the proper cell site as the signal strength of the active user changes.
TDMA is a communication technique used in cellular radio systems whereby users communicate with each other on the basis of non-overlapping transmission bursts through a common channel. Since there is no overlap, the same carrier frequency may be assigned to all users sharing the channel. Multiple access refers to techniques that allow a plurality of stations to communicate via a single channel. In TDMA, each interval of T.sub.F seconds, called a frame, is divided into discrete time slots. Each user may use one or more time slots.
The main impairments in received signals are the result of propagation effects. Multipath propagation due to scatter from obstructions within a few hundred feet of the receiving terminal, multipath propagation due to large echoes from distant but large reflectors, and shadowing of the direct path by intervening larger-scale features of the terrain each may contribute to the signal impairment. The effect of multipath propagation is a standing-wave pattern in space determined by the amplitude and phase relationships between the direct and reflected and/or scattered energy components. In a cellular system a vehicle moving through this standing wave pattern will experience short-term fluctuations in signal intensity, typically with a period of a fraction of a second. See, e.g., A. F. Inglis, Electronic Communications Handbook, Chapter 22-6, McGraw-Hill, Inc. (1988).
Multipath propagation interference significantly limits the maximum permissible data rate for a mobile RF channel. Multipath propagation in urban and suburban areas causes signal impairments that, if not counteracted, degrade signal reception and voice quality. The difficulty of dealing with these impairments is compounded by the fact that, due to the mobile nature of the application, the channel characteristics as perceived by the receiver are time-variant.
There are two major classes of impairments caused by multipath propagation: Flat (or frequency non-selective) fading and frequency selective fading. Flat fading is caused by multipath conditions in which the differential propagation delay between different paths is significant with respect to the carrier period but negligible compared to the symbol period (i.e., the duration of a transmitted symbol represented by a predefined number of bits) of the signalling scheme used. Frequency selective fading occurs when the differential propagation delay is a major fraction (&gt;25%) of the signalling or symbol period, in which case the frequency characteristics of the channel vary Within the bandwidth of the transmitted signal.
Flat fading results, e.g., when digital data is transmitted at a rate of 5 ksps (5000 symbols per second) at a carrier frequency close to 900 MHz with differential propagation delays of 10 .mu.sec or less. The differential propagation delay in this case is much larger than the carrier period but only a small fraction (5%) of the symbol period; thus the amplitude and phase of the received signal continuously change when the receiver moves under these conditions. This is because the received signal is the vector sum of the direct and multipath signals, which constructively and destructively add due to the changing path lengths. However, because the differential propagation delay is only a small fraction of the symbol period, all the frequencies in the signal of interest spectrum are attenuated more or less by the same amount, hence the name flat fading. This kind of fading is present in almost all urban and suburban areas in which multipath propagation is caused by reflections from buildings and natural obstructions.
An example of frequency selective fading would result with the same parameters as above except that the signalling rate is increased to, e.g., 25 ksps. In that case the differential propagation delay is 25% of the symbol period and results in major degradation in the performance of the receiver due to time dispersion, i.e., smearing of the received signal pulses. Time dispersion causes intersymbol interference (ISI); that is, the signal at the decision points as viewed by the receiver is made up of a superposition of contributions from multiple symbols. Hence if the receiver attempts to make a decision based upon a single received symbol, i.e., without any further processing, its performance will degrade severely since adjacent symbols will interfere with the symbol on which the receiver is making a decision.
Digital adaptive equalizers are widely used to combat the effect of time dispersion in high speed communication channels. Equalization has been used for many years in high speed data modems over analog telephone lines. The introduction of high speed digital transmission over cellular and other radio channels has mandated the use of equalization or other similar techniques that can counteract the effects of time dispersion. The equalization problem in the mobile radio channel, however, is considerably more complex than that in telephone channels, mainly because the channel impulse response in the mobile radio channel is time variant and must be estimated by the receiver in real time. Thus the receiver must not only make decisions based upon the received signal, it must also adapt the equalizer parameters in response to the fast-changing channel conditions.
As fading rate approaches the TDMA block rate, both deep energy fading and minimum-to-maximum phase channel response transitions frequently occur in a frame duration, which result in a significant number of equalizer loss-of-locks. Since most equalizers have difficulty recovering from loss-of-lock, this greatly degrades the receiver performance and has raised worldwide interest. The phenomenon is especially pronounced in narrow band TDMA systems in a mobile, as opposed to portable or indoor, environment.
FIG. 1 depicts the processing performed by a Decision Feedback Equalizer (DFE). The DFE has a forward section 1 comprising a tapped delay line fed by samples of the received complex baseband signal and a feedback tapped delay line 3 fed by the output of a decision circuit 5. The input of the decision circuit is given by the equation ##EQU1## where v.sub.k-j represents samples of the received signal, c.sub.j represents the weights of the equalizer taps, I.sub.k-j represents the signal decisions made by the decision circuit, and K.sub.1 +1 and K.sub.2 represent the respective number of forward and feedback taps. The forward section may be either synchronous, i.e., clocked at the rate of one sample per received symbol, or fractionally-spaced, i.e., clocked at multiple (typically 2) times for each received symbol. The latter is the preferred approach since it provides sampling timing insensitivity, a significant advantage in a highly dispersive time-variant environment.
The adaptation of the DFE tap weights in an environment where their optimal values change significantly within a given time slot is the subject of the present invention. Fast adapting algorithms have been published in the technical literature and are generally of the Recursive Least Square (RLS) type. Several different variants (e.g., direct Kalman, Square-root Kalman, Lattice filter, MSE Lattice) may be used with the enhanced adaption method described below.
The adaption method known as the direct form Kalman update algorithm comprises a gain update section and a tap update section. The tap update section, described by equation 6 below, is similar to more widely used techniques derived through MSE-criterion based methods, except that the gain is a vector with a length equal to the equalizer length (sum of the number of forward and feedback section taps), rather than a single scaler parameter adjusted through experimentation. Moreover, the gain is calculated in real time based on the received signal samples. The equations governing these updates are summarized below:
Compute output: EQU I.sub.N (t)=Y'.sub.N (t)C.sub.N (t-1) (2) PA1 Compute error: EQU e.sub.N (t)=I.sub.N (t)-I.sub.N (t) (3)
Compute Kalman gain vector: EQU P.sub.N (t-1)Y*.sub.N (t) (4) ##EQU2##
Update inverse of the correlation matrix: ##EQU3##
Update coefficients: ##EQU4## See J. Proakis, Digital Communications, McGraw Hill, 1989.
At the beginning of each TDMA slot in the Kalman technique the equalizer tap weights are all set to zero and the equalizer is trained, that is, the initial tap weight estimates are calculated. The tap weight calculations use a sync sequence sent by the transmitter and known in advance by the receiver. During the training period, the sync word symbols replace the decision symbols produced by the decision circuit and the adaptation algorithm described above is executed to generate updates of the tap weights, as described in equation 6. If the sync word is sufficiently long (e.g., greater than two times the equalizer length), the tap weights are adjusted to their correct values at the end of the training period, at which point the equalizer is ready to generate decisions based upon the received signal.
Processing of the signal in the reverse order from which it was received was recently introduced as a means of improving receiver performance under certain dispersive multipath conditions. A reverse processing algorithm is described in S. Ariyavisitakul, "Equalization of a Hard-Limited Slowly-Fading Multipath Signal Using a Fade Equalizer With a Time-Reversal Structure," 40th IEEE Vehicular Technology Conference, pages 520 to 526, May, 1990. That and other known algorithms, however, are incapable of working in an environment where there is a high probability of deep fade and minimum-to-maximum phase transition within a block duration. The known algorithms assume that the channel dispersion is fixed in a block duration and can be classified as either minimum phase or maximum phase. They choose either forward or reverse processing once per block by simply looking at the equalizer training status.
A TDMA communication system with adaptive equalization for reducing multipath propagation distortion is described in U.S. Pat. No. 4,852,090, which issued Jul. 25, 1989 and which is hereby incorporated by reference into the instant application. The reader is referred to the '090 patent for further background on the present invention.